1. Field of the Invention
This invention relates to apparatus and methods for reducing distortion in loudspeakers by specially constructed phase plugs.
2. Description of the Prior Art
When the diaphragms of loudspeakers are driven for high volume output, there is not only a tendency to lose more of the higher frequency sounds, but also to introduce noise as well. When waves from different regions of the diaphragm reach a common point, such as the open mouth of a horn, at different times, a high pressure wave phase will be neutralized by a low pressure wave phase from another region. This effect is more pronounced as the frequency increases. The net result is a loss of fidelity at the high end of the bandpass.
Wente in U.S. Pat. No. 2,037,187 taught a multisection transducer or phase plug comprising a family of nested concentric cones that forms a plurality of converging sound ducts of equal length. This causes sound originating in all areas of the diaphragm to reach a common point at the same time to overcome this phase distortion. Since then, phase plugs have been shown with a variety of configurations. Levy in U.S. Pat. No. 2,858,377 shows converging holes. Blackburn in U.S. Pat. No. 2,183,528 shows slots. Belisle in U.S. Pat. No. 4,776,428 shows vanes. Smith in U.S. Pat. No. 4,225,010 discusses distortion contributed by the central region of the diaphragm and teaches a damper pad positioned away from the diaphragm.